Sheet separating device with noise suppressing function and auto document feeder having such sheet separating device

ABSTRACT

A sheet separating device having a noise suppressing function is used with an auto document feeder. The sheet separating device includes a sheet separating pad, a sheet separating holder, a resilient element and a noise suppressing element. The sheet separating pad cooperates with a sheet separating roller to separate a paper sheet fed into a sheet-guiding path of the auto document feeder. The sheet separating pad is disposed on the sheet separating holder. The resilient element is sustained against or coupled to the sheet separating holder, so that sheet separating pad disposed on the sheet separating holder is urged against the sheet separating roller. The noise suppressing element is arranged on the sheet separating holder for altering the natural frequency dynamic response of an assembly of the sheet separating pad and the sheet separating holder, thereby shifting the natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder from the working frequency and the harmonic frequency of the sheet separating roller.

FIELD OF THE INVENTION

The present invention relates to a sheet separating device, and more particularly to a sheet separating device having a noise suppressing function. The present invention also relates to an auto document feeder having such a sheet separating device and a fabricating process of such a sheet separating device.

BACKGROUND OF THE INVENTION

With increasing industrial development, digitalized office technologies have experienced great growth and are now rapidly gaining in popularity. For example, a diversity of office machines such as copy machines, printers, fax machines and scanners (also referred as single-function peripherals) are utilized to achieve various purposes. The diverse office machines, however, occupy lots of space. Nowadays, a multifunction peripheral having multiple functions in one structural unit, for example the functions of a printer, a scanner, a fax machine and/or a copy machine, is thus developed. As a consequence, the processing capability of the multifunction peripheral is increased and the operative space thereof is reduced.

For successively and continuously feeding many paper sheets, the single-function peripheral or the multifunction peripheral usually has an auto document feeder (ADF). After a stack of papers to be scanned are placed on the sheet feeding tray, the sheet-feeding mechanism of the auto document feeder will successively transport the paper sheets into the inner portion of the single-function peripheral or the multifunction peripheral so as to implement associated operations such as scanning, faxing, scanning operations and the like.

FIG. 1 is a schematic cross-sectional view illustrating a conventional auto document feeder for use with an office machine. The auto document feeder 1 of FIG. 1 principally comprises a sheet feeding tray 10, a sheet feeding device 11, a sheet separating device 12 and a driving device (not shown). An example of the driving device is a driving motor. The sheet feeding device 11 is connected to the driving device. After a stack of paper sheets to be scanned are placed on the sheet feeding tray 10, the sheet feeding device 11 is driven by the driving device to successively pick the uppermost paper sheets one by one into the sheet-guiding path 13 in order to be further processed by the scanning module (not shown) of the office machine. The sheet separating device 12 is arranged at the entrance of the sheet-guiding path 13 and adjacent to the sheet feeding device 11. The sheet separating device 12 and the sheet feeding device 11 cooperate with each other to separate only a single paper sheet from the stack of paper sheets. As such, a single paper sheet is allowed to be fed into the sheet-guiding path 13 at each time.

In a case that a paper-feeding instruction is received by the auto document feeder 1, the sheet separating roller 113 of the sheet feeding device 11 is driven to rotate by the driving device. Upon rotation of the sheet separating roller 113, the sheet pick-up roller 111 is lowered. As a consequence, the sheet pick-up roller 111 is lowered to the paper-feeding position to be contacted with the uppermost paper sheet. Upon rotation of the sheet pick-up roller 111, the paper sheet is transported into the entrance of the sheet-guiding path 13 due to the friction force between the sheet pick-up roller 111 and the paper sheet.

As known, harsh noise is readily generated when the driving device is activated and the sheet separating roller 113 of the sheet feeding device 11 is rotated. In addition, after the paper sheet has been fed into the sheet-guiding path 13 and before the paper sheet is contacted with the sheet separating roller 113 and the sheet separating device 12, harsh noise is usually generated. Until the paper sheet has been transported across the region between the sheet separating roller 113 and the sheet separating device 12, the noise is reduced. In addition, after the lowermost paper sheet has been transported across the region between the sheet separating roller 113 and the sheet separating device 12, the driving device is also running and the sheet separating roller 113 is continuously rotated and contacted with the sheet separating device 12. Under this circumstance, harsh noise is still generated. Until the sheet feeding device 11 is returned to the standby position, the driving device and the sheet separating roller 113 stop rotating and thus the noise is eliminated.

Some literatures disclosed that the frictional force generated between the sheet separating roller 113 and the sheet separating device 12 is a key factor causing the harsh noise. Although the noise may be reduced by changing the material or the friction coefficient of the sheet separating device 12, the efficacy of suppressing noise is not desirable and it is hard to keep certain safety margin for separation function of the sheet separating device 12. In addition, the use of special material is not cost-effective.

Therefore, there is a need of providing a sheet separating device having a noise suppressing function so as to obviate the drawbacks encountered from the prior art.

SUMMARY OF THE INVENTION

The present invention relates to a sheet separating device having a noise suppressing function. The present invention also relates to an auto document feeder having such a sheet separating device and a fabricating process of such a sheet separating device. The sheet separating device has a simple structure and is cost-effective.

In accordance with an aspect of the present invention, there is provided a sheet separating device having a noise suppressing function to be used with an auto document feeder. The auto document feeder includes a sheet separating roller in a sheet-guiding path. The sheet separating device includes a sheet separating pad, a sheet separating holder, a resilient element and a noise suppressing element. The sheet separating pad is arranged in the sheet-guiding path and facing the sheet separating roller, and cooperates with the sheet separating roller to separate a paper sheet fed into the sheet-guiding path. The sheet separating holder has a first surface, wherein the sheet separating pad is disposed on the first surface of the sheet separating holder. The resilient element is disposed in the vicinity of the sheet separating holder and sustained against or coupled to the sheet separating holder, so that the sheet separating pad disposed on the sheet separating holder is urged against the sheet separating roller by the resilient element. The noise suppressing element is arranged on the sheet separating holder for altering the natural frequency dynamic response of an assembly of the sheet separating pad and the sheet separating holder, thereby shifting the natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder from the working frequency and the harmonic frequency of the sheet separating roller.

In accordance with another aspect of the present invention, there is provided an auto document feeder. The auto document feeder includes a sheet feeding device and a sheet separating device. The sheet feeding device includes a sheet pick-up roller and a sheet separating roller for successively transporting paper sheets into a sheet-guiding path. The sheet separating device includes a sheet separating pad, a sheet separating holder, a resilient element and a noise suppressing element. The sheet separating pad is arranged in the sheet-guiding path and facing the sheet separating roller, and cooperates with the sheet separating roller to separate a paper sheet fed into the sheet-guiding path. The sheet separating holder has a first surface, wherein the sheet separating pad is disposed on the first surface of the sheet separating holder. The resilient element is disposed in the vicinity of the sheet separating holder and sustained against or coupled to the sheet separating holder, so that the sheet separating pad disposed on the sheet separating holder is urged against the sheet separating roller by the resilient element. The noise suppressing element is arranged on the sheet separating holder for altering the natural frequency dynamic response of an assembly of the sheet separating pad and the sheet separating holder, thereby shifting the natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder from the working frequency and the harmonic frequency of the sheet separating roller.

In accordance with a further aspect of the present invention, there is provided a fabricating process of a sheet separating device of an auto document feeder. The auto document feeder includes a sheet separating roller. The fabricating process includes the following steps. Firstly, the working frequency and the harmonic frequency of the sheet separating roller are obtained. Then, a sheet separating pad and a sheet separating holder are provided. According to the working frequency and the harmonic frequency of the sheet separating roller, the overall weight of an assembly of the sheet separating pad and the sheet separating holder is adjusted or altered by a noise suppressing element, thereby changing the natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder and shifting the natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder from the working frequency and the harmonic frequency of the sheet separating roller.

The above contents of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a conventional auto document feeder for use with an office machine;

FIG. 2A is a schematic cross-sectional view illustrating an auto document feeder according to a preferred embodiment of the present invention, in which the auto document feeder is in a standby state;

FIG. 2B is a schematic cross-sectional view illustrating the auto document feeder of FIG. 2A, in which the auto document feeder is in a sheet-feeding state;

FIG. 3 is a schematic partially enlarged view illustrating the sheet separating device of the auto document feeder shown in FIG. 2;

FIGS. 4A and 4B are schematic exploded views illustrating the sheet separating device shown in FIG. 3 and taken from different viewpoints;

FIG. 5 is a schematic perspective view illustrating another exemplary noise suppressing element;

FIG. 6 is a flowchart illustrating a fabricating process of the sheet separating device of the present invention; and

FIG. 7 is a diagram showing the comparison between the amplitude versus the natural frequency dynamic response of the assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.

FIG. 2A is a schematic cross-sectional view illustrating an auto document feeder according to a preferred embodiment of the present invention, in which the auto document feeder is in a standby state. FIG. 2B is a schematic cross-sectional view illustrating the auto document feeder of FIG. 2A, in which the auto document feeder is in a sheet-feeding state. As shown in FIGS. 2A and 2B, the auto document feeder principally comprises a sheet feeding tray 20, a sheet feeding device 21, a sheet separating device 22 and a driving device (not shown). An example of the driving device is a driving motor. The sheet feeding device 21 is connected to the driving device. After a stack of paper sheets to be scanned are placed on the sheet feeding tray 20, the sheet feeding device 21 is driven by the driving device to successively pick the uppermost paper sheets 3 one by one into the sheet-guiding path 23 in order to be further processed by the scanning module (not shown) of the office machine. The sheet separating device 22 is arranged at the entrance of the sheet-guiding path 23 and adjacent to the sheet feeding device 21. The sheet separating device 22 and the sheet feeding device 21 cooperate with each other to separate only a single paper sheet from the stack of paper sheets. As such, a single paper sheet is allowed to be fed into the sheet-guiding path 23 at each time.

In this embodiment, the sheet feeding device 21 comprises a sheet pick-up roller 211, a sheet pick-up arm 212, a sheet separating roller 213 and a sheet stopper (not shown). The axle of the sheet separating roller 213 is directly or indirectly connected to the driving device, so that the sheet separating roller 213 is driven to rotate by the driving device. The sheet pick-up arm 212 is connected to the sheet pick-up roller 211 and the sheet separating roller 213. With the axle of the sheet separating roller 213 serving as a rotating shaft, the sheet pick-up arm 212 is driven by the sheet separating roller 213 or the driving device to rotate such that the sheet pick-up roller 211 is close to or distant from the sheet feeding tray 20. In some embodiment, the axle of the sheet pick-up roller 211 is sheathed by a first gear set, the axle of the sheet separating roller 213 is sheathed by a second gear set, and a transmission gear set 214 is arranged between the sheet pick-up roller 211 and the sheet separating roller 213. The transmission gear set 214 is engaged with the first gear set and the second gear set, so that the sheet pick-up roller 211 is rotated with the sheet separating roller 213. In a case that the sheet pick-up roller 211 upon rotation is contacted with the paper sheet, the paper sheet is fed into the sheet-guiding path 23. In some embodiments, the transmission gear set 214 is replaced by a transmission belt (not shown). The sheet stopper arranged at the entrance of the sheet-guiding path 23. In a case that a paper-feeding instruction is received, the uppermost paper sheet 3 is transported into the entrance of the sheet-guiding path 23 without being stopped by the sheet stopper. Whereas, in a case that no paper-feeding instruction is received, the paper sheets 3 on the sheet feeding tray 20 are hindered by the sheet stopper from being transported into the entrance of the sheet-guiding path 23.

Please refer to FIGS. 2A and 2B again. The sheet separating roller 213 is covered by a first frictional layer 213 a. Upon rotation of the sheet separating roller 213, the first frictional layer 213 a offers a frictional force required to transport the paper sheet 3 fed by the sheet pick-up roller 211 into the sheet-guiding path 23. The sheet pick-up roller 211 is covered by a second frictional layer 211 a. Upon rotation of the sheet pick-up roller 211, the second frictional layer 211 a offers a frictional force required to transport the paper sheet 3 into the entrance of the sheet-guiding path 23.

In a case that a paper-feeding instruction is received by the auto document feeder 2, the sheet separating roller 213 of the sheet feeding device 21 is driven to rotate by the driving device. Upon rotation of the sheet separating roller 213, the sheet pick-up arm 212 is lowered. As a consequence, the sheet pick-up roller 211 is lowered to the paper-feeding position where the pick-up roller 211 is in contact with the uppermost paper sheet 3. Upon rotation of the sheet pick-up roller 211, the paper sheet 3 is transported into the entrance of the sheet-guiding path 23 due to the friction force offered by the second frictional layer 211 a.

FIG. 3 is a schematic partially enlarged view illustrating the sheet separating device of the auto document feeder shown in FIG. 2. FIGS. 4A and 4B are schematic exploded views illustrating the sheet separating device shown in FIG. 3 and taken from different viewpoints. Please refer to FIGS. 3, 4A and 4B. The sheet separating device 22 comprises a sheet separating pad 221, a sheet separating holder 222, a resilient element 223 and a noise suppressing element 224. The sheet separating pad 221 is arranged at the entrance of the sheet-guiding path 23 and faces the sheet separating roller 213 of the sheet feeding device 21. The sheet separating pad 221 has a frictional surface 221 a in contact with the first frictional layer 213 a of the sheet separating roller 213 or the paper sheet that is transported between the first frictional layer 213 a of the sheet separating roller 213 and the frictional surface 221 a of the sheet separating pad 221. The frictional forces resulted from the paper sheet, the first frictional layer 213 a and the frictional surface 221 a should be elaborately controlled. It is preferred that the coefficient of friction μ1 (friction force) between the first frictional layer 213 a of the sheet separating roller 213 and the paper sheet is greater than the coefficient of friction μ2 (friction force) between the frictional surface 221 a of the sheet separating pad 221 and the paper sheet; and the coefficient of friction μ2 (friction force) is greater than the coefficient of friction μ3 (friction force) between the paper sheets. That is, μ1>μ2>μ3. As a consequence, the paper sheets can be successively separated and fed into the sheet-guiding path 23.

Preferably, the sheet separating pad 221 is made of a material selected from silicon, polyurethane (PU) rubber, ethylene propylene diene (EPDM) rubber, nitrile butadiene rubber (NBR) or other rubber. The sheet separating holder 222 is made of an insulating material. The sheet separating pad 221 is fixed on a first surface 222 a of the sheet separating holder 222 by adhesive, screwing means, clamping means or other fastening means. Optionally, a vibration-absorbing element (not shown) such as a sponge may be disposed between the sheet separating pad 221 and the sheet separating holder 222. The resilient element 223 (for example a spring) is sustained against or coupled to a second surface 222 b (for example a bottom surface) of the sheet separating holder 222. In addition, an axle 222 c is protruded from bilateral sides of an edge of the sheet separating holder 222. The axle 222 c is pivotally coupled to the internal portion of the auto automatic feeder 2 and serves as a rotating shaft of the sheet separating holder 222. Two supporting elements 222 d are extended downwardly from bilateral sides of another edge of the sheet separating holder 222 for supporting the sheet separating holder 222. An end of the resilient element 223 is fixed in the inner surface of the auto automatic feeder 2. The other end of the resilient element 223 is sustained against or coupled to the second surface 222 b of the sheet separating holder 222. Due to the elastic force generated by the resilient element 223, the sheet separating pad 221 supported by the sheet separating holder 222 is urged against the sheet separating roller 213. Consequently, the sheet separating pad 221 is contacted with the first frictional layer 213 a of the sheet separating roller 213 or the paper sheet that is transported between the sheet separating roller 213 and the sheet separating pad 221. Optionally, the sheet separating device 22 further comprises a sheet separation film 225, which is partially attached on the sheet separating pad 221 for guiding the paper to be transported between the sheet separating roller 213 and the sheet separating pad 221 or preventing no-feeding condition. For example, the sheet separation film 225 is made of poly(ethylene terephthalate) (e.g. Mylar).

The noise suppressing element 224 is used to adjust or alter the weight of the assembly of the sheet separating pad 221 and the sheet separating holder 222 in order to adjust or alter the natural frequency of the assembly of the sheet separating pad 221 and the sheet separating holder 222. As such, the natural frequency dynamic response of the assembly of the sheet separating pad 221 and the sheet separating holder 222 will be no longer overlapped with the working frequency and the harmonic frequency of the sheet separating roller 213. Under this circumstance, the resonance phenomenon is avoided and thus the noise generated during operation of the auto document feeder 2 is suppressed.

Please refer to FIGS. 3, 4A and 4B again. An example of the noise suppressing element 224 includes but is not limited to a metallic block 224 a. The noise suppressing element 224 is fixed on the second surface 222 b of the sheet separating holder 222 by adhesive, screwing means, clamping means or other fastening means. In this embodiment, the noise suppressing element 224 is fixed on the second surface 222 b of the sheet separating holder 222 by a screw 226.

FIG. 5 is a schematic perspective view illustrating another exemplary noise suppressing element 224. In this embodiment, the noise suppressing element 224 includes one or more perforations 224 b formed in the sheet separating holder 222. The one or more perforations 224 b may reduce the weight of the assembly of the sheet separating pad 221 and the sheet separating holder 222 in order to adjust or alter the natural frequency thereof.

In a case that the working frequency and the harmonic frequency of the sheet separating roller 213 is close to or overlapped with the natural frequency dynamic response of the assembly of the sheet separating pad 221 and the sheet separating holder 222, a resonance phenomenon occurs and harsh noise is generated. In accordance with a key feature of the present invention, the resonance phenomenon is avoided by changing or adjusting the overall weight of the sheet separating pad 221 and the sheet separating holder 222 in order to adjust or alter the natural frequency of the assembly of the sheet separating pad 221 and the sheet separating holder 222. As such, the natural frequency dynamic response of the assembly of the sheet separating pad 221 and the sheet separating holder 222 will be no longer overlapped with the working frequency and the harmonic frequency of the sheet separating roller 213.

FIG. 6 is a flowchart illustrating a fabricating process of the sheet separating device of the present invention. First of all, the working frequency and the harmonic frequency of the sheet separating roller are obtained by analysis and calculation (Step S11). Next, the overall weight of the assembly of the sheet separating pad and the sheet separating holder is adjusted or altered by a noise suppressing element according to the working frequency and the harmonic frequency of the sheet separating roller, thereby changing the natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder and shifting the natural frequency and dynamic response of the assembly of the sheet separating pad and the sheet separating holder from the working frequency and the harmonic frequency of the sheet separating roller (Step S12). As a consequence, the natural frequency dynamic response of the assembly of the sheet separating pad 221 and the sheet separating holder 222 will be no longer overlapped with the working frequency and the harmonic frequency of the sheet separating roller 213. In the step S12, the overall weight of the assembly of the sheet separating pad and the sheet separating holder can be previously calculated according to the working frequency and the harmonic frequency of the sheet separating roller. In some embodiments, the overall weight of the assembly of the sheet separating pad and the sheet separating holder is increased by mounting a metallic block on the second surface of the sheet separating holder. In some embodiments, the overall weight of the assembly of the sheet separating pad and the sheet separating holder is decreased by forming one or more perforations in the sheet separating holder.

In a case that the sheet separating roller is driven by the driving device (e.g. a driving motor) to rotate at a specified speed (e.g. 2,500 pps), the working frequency of the sheet separating roller is for example 53.48 Hz, and the harmonic frequency of the sheet separating roller is listed in Table 1.

TABLE 1 working frequency and harmonic frequency of the sheet separating roller (Hz) 1 2 3 . . . 7 8 9 10 Harmonic frequency 53.48 106.96 160.44 . . . 374.36 427.84 481.32 534.80

The natural frequency dynamic response of the assembly of the sheet separating pad and the sheet separating holder for various overall of the assembly is listed in Table 2.

TABLE 2 dynamic response of the assembly (Hz) weight 1 2 3 4 5 6 7 8 9 10 A 4.2 g 418 711 1102 1229 1325 1723 1753 2703 2939 3680 B 8.8 g 270 731 1055 1281 1318 1371 1487 2448 3201 3545 C 2.1 g 425 678 1013 1213 1316 1675 1742 2586 2702 3134

If the overall weight of the assembly of the sheet separating pad and the sheet separating holder is 4.2 g, the natural frequency dynamic response of the assembly is indicated in the row-A of Table 2. If the overall weight of the assembly is increased from 4.2 g to 8.8 g, the natural frequency dynamic response of the assembly is indicated in the row-B of Table 2. If the overall weight of the assembly is decreased from 4.2 g to 2.1 g, the natural frequency dynamic response of the assembly is indicated in the row-C of Table 2.

FIG. 7 is a diagram showing the comparison between the amplitude versus the natural frequency dynamic response of the assembly. Please refer to Table 1, Table 2 and FIG. 7. In a case that the working frequency and the harmonic frequency of the sheet separating roller 213 is close to or overlapped with the natural frequency dynamic response of the assembly of the sheet separating pad 221 and the sheet separating holder 222, a resonance phenomenon occurs and harsh noise is generated. For example, the eighth harmonic frequency of the sheet separating roller 213 (427.84 Hz) L₁ is close to the first-modal natural frequency dynamic response of the assembly, and thus a resonance phenomenon occurs. Under this circumstance, the change of the material type, the change of the elasticity of the resilient element and other measures fail to considerably avoid noise generation because the resonance phenomenon may enlarge the amplitude. According to the concept of the present invention, the natural frequency dynamic response of the assembly of the sheet separating pad 221 and the sheet separating holder 222 will be no longer close to or overlapped with the working frequency and the harmonic frequency of the sheet separating roller 213 by changing or shifting the natural frequency dynamic response of the assembly.

Since the eighth harmonic frequency of the sheet separating roller 213 (427.84 Hz) L₁ is close to the first-modal natural frequency dynamic response of the assembly, the efficacy of suppressing noise is undesired. In a case that the overall weight of the assembly is decreased to 2.1 g, the eighth harmonic frequency of the sheet separating roller 213 (427.84 Hz) L₁ is close to the first-modal natural frequency dynamic response of the assembly (425 Hz), and thus the efficacy of suppressing noise is also undesired. Whereas, in a case that the overall weight of the assembly is increased to 8.8 g, the first-modal natural frequency dynamic response of the assembly (270 Hz) is largely shifted from the eighth harmonic frequency of the sheet separating roller 213 (427.84 Hz) L₁, and thus the efficacy of suppressing noise is enhanced. That is, the efficacy of suppressing noise is better when the overall weight of the assembly of the sheet separating pad and the sheet separating holder is increased.

In some embodiments, the efficacy of suppressing noise is better when the overall weight of the assembly of the sheet separating pad and the sheet separating holder is decreased. In other words, the overall weight of the assembly of the sheet separating pad and the sheet separating holder is increased or decreased according to the practical requirements.

From the above description, the efficacy of suppressing noise is enhanced by changing the natural frequency dynamic response of the assembly and shifting the natural frequency dynamic response of the assembly from the working frequency and the harmonic frequency of the sheet separating roller. Since the resonance phenomenon is minimized, the noise is largely reduced during operation of the sheet separating device of the present invention. Moreover, the sheet separating device of the present invention has a simple structure and is cost-effective.

While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures. 

1. A sheet separating device having a noise suppressing function to be used with an auto document feeder, said auto document feeder comprising a sheet separating roller in a sheet-guiding path, said sheet separating device comprising: a sheet separating pad arranged in said sheet-guiding path and facing said sheet separating roller, and cooperating with said sheet separating roller to separate a paper sheet fed into said sheet-guiding path; a sheet separating holder having a first surface, wherein said sheet separating pad is disposed on said first surface of said sheet separating holder; a resilient element disposed in the vicinity of said sheet separating holder and sustained against or coupled to said sheet separating holder, so that said sheet separating pad disposed on said sheet separating holder is urged against the sheet separating roller by said resilient element; and a noise suppressing element arranged on said sheet separating holder for altering the natural frequency dynamic response of an assembly of said sheet separating pad and said sheet separating holder, thereby shifting the natural frequency dynamic response of said assembly of said sheet separating pad and said sheet separating holder from the working frequency and the harmonic frequency of said sheet separating roller.
 2. The sheet separating device according to claim 1 wherein said sheet separating holder further comprises a second face opposed to said first surface, and said resilient element is sustained against or coupled to said second surface of said sheet separating holder.
 3. The sheet separating device according to claim 2 wherein said noise suppressing element is a metallic block disposed on said second surface of said sheet separating holder for increasing the weight of said assembly of said sheet separating pad and said sheet separating holder, thereby shifting the natural frequency dynamic response of said assembly of said sheet separating pad and said sheet separating holder from the working frequency and the harmonic frequency of said sheet separating roller.
 4. The sheet separating device according to claim 3 wherein said noise suppressing element is fixed on said second surface of said sheet separating holder by a screw.
 5. The sheet separating device according to claim 1 wherein said noise suppressing element includes one or more perforations formed in said sheet separating holder for reducing the weight of said assembly of said sheet separating pad and said sheet separating holder, thereby shifting the natural frequency dynamic response of said assembly of said sheet separating pad and said sheet separating holder from the working frequency and the harmonic frequency of said sheet separating roller.
 6. The sheet separating device according to claim 1 wherein said sheet separating roller has a first frictional layer, and said sheet separating pad has a frictional surface in contact with said first frictional layer of said sheet separating roller or a paper sheet that is transported between said first frictional layer of said sheet separating roller and said frictional surface of said sheet separating pad.
 7. The sheet separating device according to claim 6 wherein the friction force between said first frictional layer of said sheet separating roller and said paper sheet is greater than the friction force between said frictional surface of the sheet separating pad and said paper sheet, and the friction force between said frictional surface of the sheet separating pad and said paper sheet is greater than the friction force between adjacent papers.
 8. The sheet separating device according to claim 1 wherein said sheet separating pad is made of a material selected from a group consisting of silicon, polyurethane (PU) rubber, ethylene propylene diene (EPDM) rubber and nitrile butadiene rubber (NBR).
 9. The sheet separating device according to claim 1 wherein said sheet separating holder is made of an insulating material.
 10. The sheet separating device according to claim 1 wherein said sheet separating pad is fixed on said first surface of said sheet separating holder by adhesive, screwing means, clamping means or fastening means.
 11. The sheet separating device according to claim 1 wherein an axle is protruded from bilateral sides of an edge of said sheet separating holder and two supporting elements are extended downwardly from bilateral sides of another edge of said sheet separating holder.
 12. The sheet separating device according to claim 1 wherein said resilient element is a spring.
 13. The sheet separating device according to claim 1 further comprising a sheet separation film, which is partially attached on said sheet separating pad.
 14. The sheet separating device according to claim 13 wherein said sheet separation film is made of poly(ethylene terephthalate).
 15. An auto document feeder comprising: a sheet feeding device comprising a sheet pick-up roller and a sheet separating roller for successively transporting paper sheets into a sheet-guiding path; and a sheet separating device comprising: a sheet separating pad arranged in said sheet-guiding path and facing said sheet separating roller, and cooperating with said sheet separating roller to separate a paper sheet fed into said sheet-guiding path; a sheet separating holder having a first surface, wherein said sheet separating pad is disposed on said first surface of said sheet separating holder; a resilient element disposed in the vicinity of said sheet separating holder and sustained against or coupled to said sheet separating holder, so that said sheet separating pad disposed on said sheet separating holder is urged against the sheet separating roller by said resilient element; and a noise suppressing element arranged on said sheet separating holder for altering the natural frequency dynamic response of an assembly of said sheet separating pad and said sheet separating holder, thereby shifting the natural frequency dynamic response of said assembly of said sheet separating pad and said sheet separating holder from the working frequency and the harmonic frequency of said sheet separating roller.
 16. A fabricating process of a sheet separating device of an auto document feeder, said auto document feeder including a sheet separating roller, said fabricating process comprising steps: obtaining the working frequency and the harmonic frequency of said sheet separating roller; providing a sheet separating pad and a sheet separating holder; and adjusting or altering the overall weight of an assembly of said sheet separating pad and said sheet separating holder by a noise suppressing element according to the working frequency and the harmonic frequency of said sheet separating roller, thereby changing the natural frequency dynamic response of said assembly of said sheet separating pad and said sheet separating holder and shifting the natural frequency dynamic response of said assembly of said sheet separating pad and said sheet separating holder from the working frequency and the harmonic frequency of said sheet separating roller.
 17. The fabricating process according to claim 16 wherein said noise suppressing element is a metallic block and said step of adjusting or altering the overall weight of said assembly of said sheet separating pad and said sheet separating holder by said noise suppressing element includes mounting said metallic block on said second surface of said sheet separating holder for increasing the overall weight of said assembly of said sheet separating pad and said sheet separating holder.
 18. The fabricating process according to claim 16 wherein said noise suppressing element includes one or more perforations and said step of adjusting or altering the overall weight of said assembly of said sheet separating pad and said sheet separating holder by said noise suppressing element includes forming said one or more perforations in said sheet separating holder for reducing the overall weight of said assembly of said sheet separating pad and said sheet separating holder. 